The invention relates to a method defined in the preamble of claim 1 for locating a mobile station (MS) in a distributed manner.
Locating methods are known which are based on the transfer of radio signals between a mobile station and mobile network. The location is determined on the basis of signals transmitted from a mobile station or from base stations and on the measurement of those signals as well as on processing the results in a substantially centralized manner.
One such method is a locating method used in the GSM system, based on the propagation time of a radio transmission and utilizing the time difference of arrival (TDOA), wherein a mobile station transmits a signal to at least three base transceiver stations (BTS) which measure the arrival times of the signals so that their time differences can be calculated. The time differences are obtained using the impulse response which is a result of correlation between a known bit pattern and a received burst signal. The bit pattern is a so-called training sequence or a corresponding known sequence. The time difference can be determined on the basis of the impulse response e.g. by selecting the point corresponding to the highest correlation or the point corresponding to the first arrived component. First arrived component refers to the signal that arrived via the shortest route in multipath propagation and the impulse response peak caused by the signal at the point corresponding to the signal. The time differences are used in a location service center (LSC) to produce at least two hyperbolas the intersection of which indicates the position of the mobile station. Because of inaccuracies in the time differences the intersection of the hyperbolas define an area, not a singular point. Positions of the hyperbolas are determined with respect to the positions of the base stations.
In a second such method a mobile station receives signals from base stations. The location is calculated in the mobile station or the measurement data, i.e. observed time difference (OTD), are sent to the mobile network where the location is then calculated.
People belonging to special groups needing emergency services, say, those suffering from heart troubles or epilepsy, nowadays have alarm devices with which they can call for immediate help. These alarm devices require special arrangements and indicate the location of the person needing help and the need for help to an emergency center. In one particular case, these devices are signalling apparatus connected to the public telephone network at the patient's home, preferably beside the bed and in the living room but also elsewhere, e.g. in the garden. As a switch in such a device is pressed, the device transmits an alarm message via the public telephone network to an emergency center. The location of the device is known in the emergency center.
A problem with the known methods is the need for signalling capacity between base stations and the location service center and the mobile station. Another problem is the computing capacity of the processor in the mobile station when the location is calculated by the mobile station. A further problem related to the indication of an emergency is brought about by the necessary special arrangements and the fact that the person using them has to keep in the vicinity of them and, when calling to an emergency center by means of a mobile station, by the fact that the caller has to know his exact location and has to communicate it verbally.
Furthermore, a problem with the known mobile-based positioning methods is that the mobile station carries out constant measurements without taking into account the surroundings and other such parameters.